It is known that, in the manufacturing process of a lithium battery, electrode materials are kneaded and, thereafter, a positive electrode medium such as lithium cobaltate is applied onto both sides of aluminum foil, which is a positive electrode, and dried and a negative electrode medium such as carbon material is applied onto both sides of copper foil, which is a negative electrode, and dried.
Here, when a battery is manufactured from the positive electrode and the negative electrode each having a respective medium applied thereto and dried (hereinafter, referred to as battery sheets), micro short-circuit occurs to deteriorate the battery performance significantly if a metallic foreign object is mixed in the battery sheet. Moreover, while the lithium battery is recently expected to be applied to electric vehicles, short-circuit might occur due to a metallic foreign object to cause catching fire or explosion and, thus, from a view point of preventing accidents caused by the lithium battery and of improving reliability, the importance of a metallic foreign object inspection with regard to a battery sheet is heightened.
As a prior art regarding a method for a foreign object inspection in an electrode material of a lithium battery, Patent Literature 1 (JP-A-2005-183142) discloses “a detection method for a foreign object causing magnetic turbulence in an electrode material for a lithium secondary battery, the method characterized in detecting existence of a foreign object causing magnetic turbulence in an electrode material for a lithium secondary battery formed in a thin film with use of a device that detects magnetic turbulence due to a magnetic impedance effect” (in CLAIMS).
Further, Patent Literature 2 (JP-A-2003-75412) discloses “a method for a defect detection for a film, the method characterized in, while applying a voltage in the thickness direction of the film with the rise time thereof being delayed, determining that the film contains a defect when a current flowing at this time exceeds a prescribed value.”